A lightweight NASA instrument from California has arrived in the Netherlands, one step closer in its journey to examine how gases escape from the nucleus of a comet.

The Microwave Instrument for the Rosetta Orbiter is one of 17 instruments that will fly aboard the European Space Agency's major mission to a comet. Rosetta will be the first spacecraft to orbit a comet, and the microwave instrument will be the first of its type to be sent to any solar system object other than Earth.

"We'll look at the abundance of the gases, their temperatures, the speed at which they're coming off, and the temperature of the comet's nucleus," said Dr. Margaret Frerking, the microwave instrument's project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The JPL-built device was incorporated into the main spacecraft structure in Alenia, Italy, and arrived in Noordwjik, Netherlands, to begin a series of tests by the European Space Agency. The next step in its journey is its path to Kourou, French Guinea, for its January 2003 launch into space. Rosetta will swing near Earth and two large asteroids before reaching its chosen dance partner, Comet Wirtanen, on Nov. 28, 2011. At that point in Wirtanen's 5.5-year orbit, the comet will be at about as far from the Sun as Jupiter and five times as far from the Sun as Earth.

Rosetta will drop a lander onto Wirtanen's nucleus, and the orbiter will circle the comet at distances as close as 2 kilometers (1.2 miles).

From the orbiter, the microwave instrument will monitor how the release of vapors from the comet's icy nucleus changes as Wirtanen moves closer to the Sun. Gases and dust escaping from the surface of a comet form a cloud-like "coma" around the nucleus and a tail pointed away from the Sun.

"The spacecraft will remain in orbit around Wirtanen for 20 months as the comet moves in from Jupiter's distance from the Sun to about Earth's distance," said JPL's Dr. Samuel Gulkis, principal investigator for the instrument. "During that time, the nucleus will warm significantly, and we'll be able to watch the whole process as the comet evolves from an inactive iceball to having a fully developed coma."

The instruments onboard the orbiter will include a camera to study surface details, a microscope to analyze dust grains coming off the nucleus, spectrometers to examine surface and coma materials in various wavelengths, and an experiment to probe the comet's interior with radio waves.

The microwave instrument is a very high frequency radio spectrometer, weighing about 20 kilograms (44 pounds). It is designed for studying water, carbon dioxide, ammonia and methanol gases, four of the most abundant gases from comets. The device is sensitive to slight differences in emission wavelengths from those gases, allowing it to measure the quantities coming off the nucleus, along with their temperatures and speeds.

"We want to get a good estimate of the amount of mass being lost by the comet so we can play that backward to get at what the comet was like shortly after it was formed," Gulkis said. That will help pin down ideas about how comets and planets were produced during the infancy of our solar system.

The microwave instrument will also be able to measure both the surface temperature of the nucleus and the temperature just below the surface. "That temperature difference will tell us about the insulating properties of the surface and help us understand the thermal physics of what's going on inside the nucleus," Gulkis said.

As Rosetta passes the stony asteroid Otawara and the carbon-rich asteroid Siwa on its roundabout route to Wirtanen, the microwave instrument will examine thermal properties of those minor planets' surfaces and check whether they have any permafrost layer leaking small quantities of water vapor into space.

Online information is available about Rosetta at http://sci.esa.int/rosetta and about the microwave instrument at http://mirowww.jpl.nasa.gov. JPL, a division of the California Institute of Technology in Pasadena, manages the instrument for NASA's Office of Space Science, Washington, D.C.